Ice cream or frozen custard machines, as well as other systems for cooling or freezing food stuffs, condiments, or other materials, typically include an evaporator situated proximate the material being chilled. For example, in ice cream machines, liquid ice cream (e.g., the mix) is typically inserted in a freezing chamber or barrel associated with the evaporator and is removed from the barrel as solid or semi-solid ice cream. The evaporator removes heat from the freezing chamber as a liquid refrigerant, such as, FREON.RTM., ammonia, R-404a, HP62, or other liquid having a low boiling point, changes to vapor in response to the heat from the liquid ice cream. Typically, the evaporator is partially filled with vapor as the liquid refrigerant boils (e.g., becomes vapor) in the evaporator.
Since most heat transfer occurs when the liquid refrigerant is changed to vapor, the partially filled evaporator is less efficient than a flooded evaporator (e.g., an evaporator filled entirely with liquid refrigerant). The partially filled evaporator also tends to unevenly cool the ice cream because the parts of the evaporator which are filled with vapor are not able to cool as effectively as the parts of the evaporator filled with liquid. Further, prior art ice cream machines are disadvantageous because the temperature does not remain constant in the evaporator due to the accumulation of vapor. The inefficiencies resulting from the partially filled evaporator require a larger, more expensive, and less energy-efficient compressor. The goal of an efficient evaporator is to reduce the quantity of vapor in the barrel to optimize the surface area for liquid refrigerant evaporation. Although there is always a quantity of vaporized refrigerant in the barrel it is essential to minimize stagnation of the vapor within the heat exchange area. By reducing the stagnation of the vaporized refrigerant within the barrel, there is a more efficient transfer of heat. There can be a closer relationship of refrigerant evaporating temperature to ice cream or frozen custard freezing temperature. A result of this closer temperature difference is higher compressor efficiency.
In addition, custard or ice cream quality and efficient manufacture of such custard or ice cream are dependent upon maintaining a constant evaporator temperature (e.g., constant barrel temperature). The barrel temperature must be kept in a proper range for making custard or ice cream so the custard or ice cream. If the custard or ice cream is allowed to become too cold, the mix or liquid ice cream in the evaporator becomes highly viscous and can block the travel of the ice cream through the barrel. Blockage of the barrel in the freezing process is commonly known as "freeze up".
Maintaining the temperature of the barrel at a constant level is particularly difficult, as ice cream flow rates through the machine vary and change the cooling load on the evaporator. For example, more heat dissipation is required as more ice cream is produced (i.e., the flow rate is increased). Additionally, if the barrel temperature is too low, refrigerant flood-back problems can adversely affect the operation of the compressor. For example, if the refrigerant is not fully evaporated as it reaches the compressor, the liquid refrigerant can damage the compressor.
Thus, there is a need for an ice cream machine which can more efficiently and more evenly cool ice cream or custard in the freezing chamber. Further still, there is a need for an ice cream machine which is not susceptible to freeze ups or to refrigerant flood-back problems.